Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature.
Various types of photoconductive insulating elements are known for use in electrophotographic imaging processes. In many conventional elements, the active components of the photoconductive insulating composition are contained in a single layer composition. This composition is typically affixed, for example, to a conductive support during the electrophotographic imaging process.
Among the many different kinds of photoconductive compositions which may be employed in typical single active layer photoconductive elements are inorganic photoconductive materials such as vacuum evaporated selenium, particulate zinc oxide dispersed in a polymeric binder, homogeneous organic photoconductive compositions composed of an organic photoconductor solubilized in a polymeric binder, and the like.
Another especially useful photoconductive insulating composition which may be employed in a single active layer photoconductive element are the high-speed "heterogeneous" or aggregate photoconductive compositions described in Light, U.S. Pat. No. 3,615,414 issued Oct. 26, 1971 and Gramza et al, U.S. Pat. No. 3,732,180 issued May 8, 1973. These aggregate-containing photoconductive compositions have a continuous electrically insulating polymer phase containing a finely divided, particulate, co-crystalline complex of (i) at least one pyrylium-type dye salt and (ii) at least one polymer having an alkylidene diarylene group in a recurring unit.
Recently, an especially useful "multi-active," photoconductive insulating composition has been developed which contains a charge-generation layer in electrical contact with a charge-transport layer, the charge-generation layer comprising a multi-phase aggregate composition as described in U.S. Pat. No. 3,615,414 having a continuous, polymeric phase and dispersed in the continuous phase a co-crystalline complex of (i) a pyrylium-type dye salt, such as 2,4,6-substituted thiapyrylium dye salt, and (ii) a polymer having an alkylidene diarylene group as a repeating unit, and the charge-transport layer comprising an organic photoconductive charge-transport material. When a uniform-polarity electrostatic charge is applied to the surface of this multi-active element and the charge-generation layer thereof is subjected to an image wise exposure to activating radiation, the charge-generation layer generates charge carriers, i.e., electron-hole pairs, and injects them into the charge-transport layer which accepts and transports these charge carriers through the multi-active element to form an electrostatic charge pattern at or near the surface of the multi-active element corresponding to the imagewise exposure. The above-described, multi-active element is described in Berwick et al, copending U.S. Pat. application Ser. No. 534,979, filed Dec. 20, 1974, now abandoned.
In the past, in various publications, such as U.S. Pat. No. 3,037,861 issued Nov. 22, 1966 and in U.S. Pat. Nos. 3,287,113 through 3,287,123 issued Apr. 19, 1966, it has been disclosed that various protonic acids (sometimes also referred to as electron acceptors, Lewis acids, or Bronsted acids) may be incorporated as chemical sensitizers or "activators" in organic photoconductive compositions, particularly homogeneous compositions composed of an organic photoconductor(s) solubilized in a polymeric binder.
In addition, in other literature publications it has been disclosed that when various protonic acids are added to conventional photoconductive compositions, such as various dye-sensitized homogeneous organic photoconductive compositions containing certain polymeric organic photoconductors such as poly(vinyl carbazoles), poly(acryl phenothiazines), etc. or certain monomeric organic photoconductors such as monomeric dialkyl aromatic amines, one can impart a persistent conductivity effect (sometimes referred to as a memory effect) to the photoconductive composition. In this regard, one may refer to such publications as Y. Hayashi, M. Kuroda, and A. Inami in Bull. Chem. Soc. Japan, 39, 1660, (1966); U.S. Pat. No. 3,512,966; and Williams, Pfister, and Abkowitz in Tappi, 56, 129 (1973).
The persistent conductivity effect referred to above has reference to the property of a photoconductive composition which possesses such a capability to generate an electrical image, i.e., an electrostatic charge image, in response to a single imaging cycle, e.g., upon being subjected to a single imagewise exposure in the presence of an electrical field, which electrical image persists over a time period sufficient to produce (from that one electrical image) a plurality of image copies. In terms of conventional electrophotographic transfer processes, this means that a single electrical image produced by an imagewise exposed photoconductive composition must have a lifetime sufficient to provide a developable background to charge image differential over a plurality of subsequent process cycles without re-exposing the photoconductive composition to the original radiation image pattern. For example, a photoconductive composition which exhibits persistent conductivity can be given an initial uniform electrostatic charge and exposed to an initial imagewise radiation pattern to form a latent electrical image. This latent electrical image can then be developed by application of a suitable electrographic developer into a visible electrographic toner image. The resultant toner image can then be transferred to a receiver sheet to form a first copy corresponding to the original imagewise exposure. The photoconductive composition bearing the original latent electrical image (by virtue of the persistent character of this electrical image) can then be re-charged by application of an electrical field, e.g., by application of a uniform electrostatic charge, and, in the absence of any imagewise re-exposure, one obtains a developable, latent electrical image corresponding to the original imagewise exposure so that a second copy of the original imagewise exposure can be generated.
In a photoconductive composition exhibiting ideal persistent conductivity properties, one would hope to be able to generate a number of copies, e.g., 5 to 50 or more, using only a single imagewise exposure. In addition, one would hope to be able to obtain a persistent electrical image having a 5 to 50 copy lifetime without having to use extremely high energy radiation exposure levels to form the original persistent electrical image pattern. That is, one would like to be able to use light radiation sources having an energy output similar to that of radiation sources used in conventional electrophotographic reproduction devices. Of course, another desirable property of a photoconductive composition intended for use as persistent conductivity medium is that the composition be capable of reuse. For example, once one has obtained the desired number of copies from the persistent electrical image pattern formed by this composition, one would like to be able to erase this electrical image and then reuse the photoconductive composition to form additional persistent electrical images.
To date, the art has claimed some success in obtaining persistent conductivity image patterns with certain types of single active layer "homogeneous" organic photoconductive compositions (i.e., photoconductive compositions composed of a solid solution of organic photoconductor and binder) by incorporating therein a dyestuff and an activator selected from the group consisting of organic carboxylic acids, nitrophenols, nitroanilines, and carboxylic acid anhydrides. (See U.S. Pat. No. 3,512,966 noted above.) Unfortunately, however, it has been found that when incorporation of the same or similar types of activators is attempted with single layer photoconductive elements containing the aforementioned high-speed, "heterogeneous" or "aggregate" photoconductive compositions described in Light, U.S. Pat. No. 3,615,414, issued Oct. 26, 1971 and Gramza et. al. U.S. Pat. No. 3,732,180 issued May 8, 1973, the resultant, single active layer, aggregate photoconductive composition exhibits little or no persistent conductivity capability.